Berislav Perić

Bis(Amino Acid) Oxalyl Amides as Ambidextrous Gelators of Water and Organic Solvents. Supramolecular Gels With Temperature Dependent Assembly-Dissolution Equilibrium

Bis(LeuOH) (1a), bis-(ValOH) (2a) and bis(PhgOH) (5a) (Phg denotes (R)-phenylglycine) oxalyl amides are efficient low molecular weight organic gelators of various organic solvents and their mixtures as well as water, water/DMSO, and water/DMF mixtures. The organisational motifs in aqueous gels are dominated primarily by lipophilic interactions while those in organic solvents are formed by intermolecular hydrogen bonding. Most of the gels are thermoreversible and stable for many months. However, 2a forms unstable gels with organic solvents which upon ageing transform into variety of crystalline shapes. For some 1a/alcohol gels, a linear correlation between alcohol dielectric constants (epsilon) and gel melting temperatures (Tg) was found. The 1H NMR and FTIR spectroscopic investigations of selected gels reveal the existence of temperature dependent network assembly/dissolution equilibrium. In the 1H NMR spectra of gels only the molecules dissolved in entrapped solvent could be observed. By using an internal standard, the concentration of dissolved gelator molecules could be determined. In FTIR spectra, the bands corresponding to network assembled and dissolved gelator molecules are simultaneously present. This enabled determination of the Kgel values by using both methods. From the plots of InKgel versus 1/T, the deltaHgel values of selected gels have been determined (-deltaHgel in 10-36 kJ mol(-1) range) and found to be strongly solvent dependent. The deltaHgel values determined by 1H NMR and FTIR spectroscopy are in excellent agreement. Crystal structures of 2a and rac-5a show the presence of organisational motifs and intermolecular interactions in agreement with those in gel fibres elucidated by spectroscopic methods.

Octahedral niobium cluster-based solid state halides and oxyhalides: effects of the cluster condensation via an oxygen ligand on electronic and magnetic properties

The influences of an oxygen ligand on the structural, magnetic and electronic properties of octahedral niobium cluster-based oxides and oxychlorides are reported. The Nb6 metal cluster is edge-bridged by twelve inner ligands and additionally bonded to six apical ligands to form Nb6Li12La6 units (L = Cl, O) wherein oxygen and chlorine are perfectly ordered. Oxygen favours the interconnection of clusters via double Oi–a/Oa–i bridges in a similar way to the double Si–a/Sa–i bridges found in Chevrel phases based on face capped Mo6Li8La6 units. Periodic density functional theory (DFT) calculations confirm that increasing the number of inner oxygen ligands at the expense of chlorine atoms favours the 14 metal-electron (ME) count per octahedral cluster unit. It is also shown that weak interactions occur between neighbouring clusters. Indeed, magnetic measurements performed on AxNb6Cl12O2 (A = Rb, x = 0.816(8); A = Cs, x = 1) series containing 15-ME species evidence antiferromagnetic interactions at low temperatures. Broken-symmetry DFT calculations of exchange parameters within spin dimer analysis confirm the experimental results.

High-Temperature Experimental and Theoretical Study of Magnetic Interactions in Diamond and Pseudo-Diamond Frameworks Built up from Hexanuclear Tantalum Clusters

Magnetic interactions in solid-state tantalum cluster compounds have been evidenced by using magnetic susceptibility measurements and corroborated by broken-symmetry DFT calculations. The three selected compounds are based on [Ta(6)X(12)(H(2)O)(6)](3+) (X=Cl or/and Br) units with edge-bridged Ta(6) octahedral clusters. Although two of them crystallise in the tetragonal space group I4(1)/a, all compounds exhibit a similar arrangement of paramagnetic clusters related to the diamond structural framework (Fd ̅3m space group). Magnetic parameters were fitted by using the [5,4] Padé approximant of high-temperature series expansion of susceptibility for the Heisenberg model (S=1/2) in the diamond framework, assuming only nearest-neighbouring interactions. Such a model appropriately describes magnetic-susceptibility measurements at temperatures T>0.7|J|/k. The magnetic interaction parameter J between two [Ta(6)Cl(12)(H(2)O)(6)](3+) clusters is estimated to be -64.28(7) cm(-1) ; it has been enhanced by replacing several chlorine inner ligands with bromine ones (J=-123(3) cm(-1) for two [Ta(6)Br(7.7(1))Cl(4.3(1))(H(2)O)(6)](3+) clusters) and is strongest between two bromine [Ta(6)Br(12)(H(2)O)(6)](3+) clusters with a value of -155(1) cm(-1) . Broken-symmetry DFT calculations within spin-dimer analysis confirmed this trend. Those interactions can be explained on the basis of the overlap between singly occupied a(2u) orbitals localised on neighbouring clusters.

Bis(tetramethylammonium) hexaaquadodeca-μ-bromo-octahedro-hexatantalum tetrabromide dihydrate

The novel title compound, [(CH(3))(4)N](2)[Ta(6)Br(12)(H(2)O)(6)]Br(4) x 2H(2)O, with a [Ta(6)Br(12)]2+ cluster unit, has been prepared and structurally characterized. The compound crystallizes in space group C2/c, with a twofold axis passing through the cluster and the centre of symmetry located between the clusters. The nearest neighbouring cluster units are aligned along the crystallographic c axis, forming a one-dimensional chain pattern.

Solid-State NMR/NQR and First-Principles Study of Two Niobium Halide Cluster Compounds

Two hexanuclear niobium halide cluster compounds with a [Nb6X12](2+) (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable Bo field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of (95)Mo nuclei in structurally related compounds containing [Mo6Br8](4+) cluster cores. Experimentally determined δiso((93)Nb) values are in the range from 2,400 to 3,000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (Vzz and δ33) coinciding with the molecular four-fold axis of the [Nb6X12](2+) unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (Vzz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (Vyy) and the largest CS principal axis (δ11) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by (79/81)Br NQR and (35)Cl solid-state NMR measurements.

Supramolecular assembling using synthons with NH—CO(S)—CS—NH and NH—CO—CO—NH functionalities: crystal structures of (S,S)-N,N′-monothiooxalyldileucine methyl ester and its dithio analogue

To compare the structural properties of oxalamide and thiooxalamide groups in the formation of hydrogen bonds suitable for supramolecular assemblies a series of retropeptides was studied. Some of them, having oxalamide bridges, are gelators of organic solvents and water. However, retropeptides with oxygen replaced by the sp(2) sulfur have not exhibited such properties. The crystal structures of the two title compounds are homostructural, i.e. they have similar packing arrangements. The monothio compound crystallizes in the orthorhombic space group P2(1)2(1)2(1) with two molecules in the asymmetric unit arranged in a hydrogen-bond network with an approximate 4(1) axis along the crystallographic b axis. However, the dithio and dioxo analogues crystallize in the tetragonal space group P4(1) with similar packing patterns and hydrogen-bonding systems arranged in agreement with a crystallographic 4(1) axis. Thus, these two analogues are isostructural having closely related hydrogen-bonding patterns in spite of the different size and polarity of oxygen and sulfur which serve as the proton acceptors.

Hexa­aqua­dodeca-μ-bromo-octahedro-hexatantalum bromide chloride octahydrate

The title compound, [Ta(6)Br(12)(H(2)O)(6)](Br(0.4)Cl(1.6)) x 8H(2)O, crystallizes in space group P 1 macro. The structure contains two crystallographically independent [Ta(6)Br(12)(H(2)O)(6)](2+) cluster cations forming distinct layers parallel to the ab plane. The compound is isoconfigurational with the double salts [Ta(6)Br(12)(H(2)O)(6)]X(2) x trans-[Ta(6)Br(12)(OH)(4)(H(2)O)(2)] x 18H(2)O (X = Cl, Br).

Bis(2,2'bipyridine-κ2N, N')(oxalato-κ2O, O')cobalt(II) pentahydrate

The synthesis and crystal structure of the mononuclear title compound, [Co(C(2)O(4))(C(10)H(8)N(2))(2)].5H(2)O, is reported. The Co atom is six-coordinated by two O atoms of a bidentate oxalate group and by four N atoms of two bipyridine ligands. The neutral [Co(C(2)O(4))(C(10)H(8)N(2))(2)] entities are connected by pi-pi stacking interactions of the aromatic systems into a two-dimensional layer, interconnected through a ladder-like hydrogen-bonding pattern of solvate water molecules.

Intermolecular contacts in the crystal packing of 2,2'-(N,N'-oxalyldiimino)bis(3-phenylpropanamide) dimethyl sulfoxide solvate.

In the title compound, C20H22N4O4*C2H6OS, two distinct hydrogen-bond systems connect oxalamide groups in one pattern and primary amide groups in the other to form a two-dimensional network perpendicular to the c axis. These hydrophilic layers are joined to the three-dimensional structure through C--H...pi interactions. The hydrogen-bonded waved layers shape holes which are occupied by disordered dimethyl sulfoxide solvent molecules.